Little things: making and bringing a cup of coffee to him/her in the morning, filling his/her car up with gas, taking care of little tasks that need done proactively. For example I had two boxes to mail out to my family. My husband mailed them without me even asking. He will take our donation box to Goodwill, make sure the house is dark and quiet when I get a migraine.
We both go out of our way to make life more enjoyable for the other.